An electrophotographic printer known in the art has developing devices detachably mounted therein. Each developing device includes a developing frame, a developing roller and a supply roller that are supported in the developing frame.
One such developing device that has been proposed is a developer cartridge that includes a cartridge frame, a developing roller rotatably supported in the cartridge frame, and a supply roller rotatably supported in the cartridge frame while confronting and contacting the developing roller. Receiving grooves extending in the general front-rear direction are formed in both left and right side walls of the cartridge frame for receiving ends of rotational shafts provided in the developing roller and supply roller, and a rib vertically intersecting the groove in the right side wall through the approximate front-rear center thereof so as to bridge the top and bottom of the groove.